Is my horse a Headshaker?

years there have been many theories as to why idiopathic headshaking occurs but recent work has helped advance our understanding of this condition
Trigeminalmediated headshaking
Recent research has determined that for many idiopathic headshakers there is sensitisation of the branches of the main nerve trigeminal nerve transmitting sensory information heat pressure and touch from the horses face to the brain.  The result is a normally innocuous stimuli such as wind on the face is perceived by the affected horse as a sharp unremitting pain.  This theory is supported by the finding that using a local anaesthetic injected around the trigeminal nerve can temporarily relieve symptoms of headshaking and that  the stimulus required for the trigeminal nerve to respond has been found to be lower in affected horses.
Damage from previous viral infection?
Its been suggested that equine herpes virus or other viral agents may cause damage to the trigeminal nerve however recent work has found no evidence of structural changes in the nerve to support this long held suspicion.
Allergies and pollen?
Due to the typical seasonal appearance of headshaking it has long been thought that allergy is the cause of headshaking. Whilst some horses may suffer from true allergy causing them to headshake antiallergy medications such as corticosteroids and antihistamines are frequently ineffective at relieving signs of headshaking. This suggests that whilst pollen and dust may play a role in the syndrome it is likely to be more complicated than a straightforward allergic response.
Exercise
It frequently noted headshakers are worse at exercise.  This may occur as part of the trigeminalmediated syndrome where wind flow in the nostrils or perhaps the changes in blood flow which occur with exercise cause pain in an already sensitised nerve.
Bright sunlight
Bright sunlight can be a trigger for headshaking behaviour which could explain why there are higher incidences of headshaking in the summer. It is thought this may occur as the light indirectly increases the sensory input to an already sensitised nerve.
Careful observation of the circumstances in which symptoms occur in an individual horse can help your vet to identify their individual trigger factors and determine the best therapeutic approach.
How can I help my headshaker? 
It is important to work with your vet firstly to determine if there is pain and if so  does it arise from the face or from elsewhere in the body? Once this is determined work can begin to establish if there is an underlying cause for facial